1. Field of the Invention
The present invention relates to a display device, and more particularly, to a display device comprising an element using a semi-amorphous thin film semiconductor. The invention further relates to an electronic apparatus using the display device.
2. Description of the Related Art
In recent years, Internet has been widely used with the development of communication technologies. It is expected that moving pictures and larger amount of information are transmitted in the future. In view of this, personal computers have been popularized for private use and on business, and a large sized display device such as a liquid crystal television has also been produced in quantities and popularized.
Among the display devices, a display device using a thin film transistor (hereinafter referred to as a TFT), such as a liquid crystal display device in particular, has been manufactured actively. An active matrix display device using a TFT can exhibit a higher image quality in contrast and gray scale levels as compared with a passive display device.
In such a display device using a TFT, a TFT whose channel forming region comprises an amorphous semiconductor (hereinafter referred to as an amorphous TFT) is widely used. A display device using an amorphous TFT displays images by using an inverted staggered TFT formed on a glass substrate and controlling pixels of the display device each including the TFT.
FIG. 4A is a plan view of a liquid crystal display device using an amorphous TFT. In FIG. 4A, a conventional liquid crystal display device comprises an amorphous TFT substrate 401, a counter substrate 402, a pixel portion 403, source signal line driver LSIs 405, gate signal line driver LSIs 404, FPCs 406, and the like. The signal line driver LSIs 404 and 405 comprise single crystalline LSIs and mounted on the amorphous TFT substrate 401. Signals are inputted from outside to the signal line driver LSIs 404 and 405 via the FPCs 406. Although the LSIs are mounted on the amorphous TFT substrate 401 in FIG. 4A, they may be mounted on the FPCs.
FIG. 4B shows a cross sectional structure taken by cutting along a dotted line of FIG. 4A. A liquid crystal is disposed between the amorphous TFT substrate 401 and the counter substrate 402 and sealed with a sealing member 407.
The aforementioned liquid crystal display device using an amorphous TFT has a problem that the property of a transistor, for example a mobility or a threshold value, is inferior to that of a transistor using single crystalline silicon.
For example, when comparing the mobility of an N-channel single crystalline transistor with an amorphous TFT, the former has a mobility of 600 to 800 cm2/Vs, whereas the latter has a mobility of about 0.5 cm2/Vs. Thus, the electrical property of the amorphous TFT is 1/1000 that of the single crystalline transistor, and therefore, it cannot make up an electrical circuit as free as the single crystalline transistor. The amorphous TFT is capable of driving pixels but not signal lines in a liquid crystal display device.
Accordingly, in a liquid crystal display device using an amorphous TFT, a driver circuit for driving signal lines is made up of LSIs using single crystalline transistors. The LSIs can drive the signal lines, however, the driver circuit has to be attached externally or connected to a glass substrate, leading to defects such as increase in the cost of implementation, and lowered reliability in a connecting part. On the other hand, a display device in which pixels and a driver circuit are integrally formed on a glass substrate by using a polysilicon TFT has been developed. A polysilicon TFT exhibits a mobility of about 100 to 200 cm2/Vs, thus a driver circuit can be formed integrally. In order to form a polysilicon TFT, however, manufacturing steps for laser crystallization, heat treatment, doping and the like are additionally required. Therefore, a glass substrate can not be made larger due to limitations of the manufacturing equipment and the costs are increased as compared with an amorphous TFT.
In view of the foregoing, pixels, a signal line driver circuit, and a gate signal line driver circuit in particular may be integrally formed by using a semi-amorphous semiconductor (hereinafter referred to as an SAS) so that external driver circuits and connecting parts thereof are reduced and the cost of implementation and reliability in the connecting parts are improved. However, a threshold voltage of an SAS TFT is higher than that of a polysilicon TFT, therefore, the amplitude for driving a signal line has to be made larger and a power supply voltage has to be made higher in the case of forming a pixel by using the SAS, leading to higher power consumption.
FIG. 2 shows an example of a conventional gate signal line driver circuit. In FIG. 2, a shift register 201 comprises clocked inverters 202 and 203, and an inverter 204. The driver circuit comprises the shift register 201, a NAND 207, a NOR 208, and buffering inverters 209 and 210, and it drives gate signal lines G1, G2, . . . , Gy in sequence.
As for a display device for monitoring, image persistence becomes a problem of concern when it is used for long periods of time. Image persistence may occur, more or less, in any of display devices. In a self-light emitting display device, luminance decays as a light emitting layer degrades. In particular, when the self-light emitting display device continues to display a fixed image, luminance decays only in a light emitting area, and thus the preceding image remains when displaying a solid image thereafter. In a liquid crystal display device also, a liquid crystal material degrades when a fixed image continues to be displayed, thus the image remains when the subsequent image is displayed.
In order to avoid such a problem, a monitor screen is turned off and partial display is performed when a user does not use the monitor screen for a certain period. FIGS. 3A and 3B show an example of this case. In FIG. 3A, a normal image is displayed on a screen whereas only time is displayed on a part of the screen in FIG. 3B. Such a partial display mode prevents the occurrence of image persistence.
However, even in a partial display mode, a conventional gate signal line driver circuit continues to be driven, which consumes as much power as in a normal display mode.